Recently, regulation of automobile running noise has become strict, and there is a great demand for such a tire making less noise as well as being improved in running stability and durability under high speed running.
Hitherto, in order to reduce a running noise of a tire (coasting noise), efforts have been made mainly to improve the tread pattern using a method called pitch-variation. Recently, however, it has become very difficult to reduce the running noise by improving the tread pattern alone. For example, between a low noise tire of which running noise is well reduced by improving the tread pattern and a smooth tire which has a smooth tread surface (a tread pattern is not provided), the difference in noise level is very small.
As a result of studies performed, it was discovered that the tread portion of a tire vibrates during running, and the tread portion itself makes a noise, and further the vibration excite excites the air existing in tread grooves to make a noise called air resonance noise, and that the vibration mode is closely related to the belt disposed in the tread portion.
In a belted radial tire which is provided in the tread portion with a steel cord cross ply belt, one antinode is formed on each side of the time equator at a specific frequency, and this frequency corresponds to the frequency at which the peak of the running noise frequency spectrum lies.
In the tire sizes for passenger car, light van and the like, the peak usually lies in the range of 800 to 1000 Hz as shown in FIG. 7, and the amplitude of vibration becomes maximum around the quarter point (p) as shown in FIG. 5, which point (p) is spaced apart from the tire equator (c) by a distance of 1/4 times the tread width.
Incidentally, the above-mentioned vibration mode pattern was obtained as follows. A test tire was vibrated by applying a vibrator to the tread shoulder portion, and the vibration was measured at 10 mm intervals using an acceleration pickup. Then, the data were processed using a Frequency analyzer and a computer to show graphically as shown in FIGS. 4-6.